1. Field of the Invention
The present invention relates to an exhaust gas recirculation (EGR) apparatus for engine to allow part of exhaust gas discharged from an engine to an exhaust passage to flow in an intake passage to recirculate back to the engine.
2. Related Art
Conventionally, a technique of the above type is employed in a vehicle engine. An exhaust gas recirculation (EGR) apparatus is arranged to introduce part of exhaust gas after combustion, which is discharged from a combustion chamber of an engine to an exhaust passage, into an intake passage through an EGR passage so that the exhaust gas is mixed with intake air flowing in the intake passage and flows back to the combustion chamber. EGR gas flowing in an EGR passage is regulated by an EGR valve provided in the EGR passage. This EGR can reduce mainly nitrogen oxide (NOx) in the exhaust gas and improve fuel consumption during a partial load of the engine.
Exhaust gas from the engine contains no oxygen or is in an oxygen lean state. Thus, when part of the exhaust gas is mixed with the intake air by EGR, the oxygen concentration of the intake air decreases. In a combustion chamber, therefore, fuel burns in a low oxygen concentration. Thus, a peak temperature during combustion decreases, thereby restraining the occurrence of NOx. In a gasoline engine, even when the content of oxygen in intake air is not increased by EGR and a throttle valve is closed to some degree, it is possible to reduce pumping loss of the engine.
Herein, recently, it is conceivable to perform EGR in the entire operating range of the engine in order to further improve fuel consumption. Realization of high EGR is thus demanded. To realize the high EGR, it is necessary for conventional arts to increase the internal diameter of an EGR passage or increase the opening area of a flow passage provided by a valve element and a valve seat of an EGR valve.
Meanwhile, as the EGR valve, an electrically-operated EGR valve is used, in which a valve element is opened and closed by a step motor to control small or fine opening position. In this type of EGR valve, a problem may occur in which foreign particles such as carbon particles in exhaust gas or hard foreign matters such as cutting chips are lodged or caught between the valve seat and the valve element. In case the foreign matters are lodged between the valve seat and the valve element, the valve element is not allowed to completely seat on the valve seat. This may cause step-out of the step motor and hence failures of EGR control. An amount of foreign matters lodged between the valve element and the valve seat may be increased as the opening area of a flow passage defined by the valve element and the valve seat is increased for high EGR.
In association with such lodging of foreign matters, an EGR apparatus disclosed in Patent Document 1 listed below is arranged to open, before start of engine cranking (before the start of engine starting), the valve element of the EGR valve at least once to a fully opened position to perform a removing operation to make foreign matters fall down from the valve seat. To be concrete, the valve element is moved from a fully closed position to a fully opened position and thereafter returned to the fully closed position. It is conceivable that this operation of “Fully-closed→Fully-opened→Fully-closed” is repeated several times.